net: dsa: qca8k: move mib struct to common code

The same MIB struct is used by drivers based on qca8k family switch. Move
it to common code to make it accessible also by other drivers.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>